Abnormal heart beats or cardiac arrhythmias can cause significant morbidity and mortality. These arrhythmias arise from a variety of causes, including atherosclerotic heart disease, ischemic heart disease, metabolic or hemodynamic derangements, rheumatic heart disease, cardiac valve disease, certain pulmonary disorders and congenital etiologies.
The normal heart rate is about 60 to 100 beats per minute. Arrhythmias, also called dysrhythmias, include tachycardias at rates exceeding 100 beats per minute for a duration of at least 3 beats. Tachycardias are also referred to as tachyarrhythmias. Sometimes no treatment is required, such as in the tachycardia following a physiologic response to stress or exercise. However, in some cases, treatment is required to alleviate symptoms or to prolong the patient's life.
A variety of treatments exists, including electric direct current cardioversion, pharmacologic therapy with drugs such as quinidine, digitalis, and lidocaine, treatment of an underlying disorder such as a metabolic derangement, and ablation by either percutaneous (closed chest) or surgical (open chest) procedures. Treatment by ablation involves destruction of a portion of cardiac tissue which displays an abnormal electrical function.
For clinical information concerning ablation therapy for supraventricular tachycardia, ventricular tachycardia and junctional ablation, see the following articles: Scheinman, Melvin M., "Ablation Therapy for Patients with Supraventricular Tachycardia," Ann. Rev. Med. 37:225-233 (1986); Hartzler, Geoffrey O., "Electrode Catheter Ablation of Refractory Focal Ventricular Tachycardia," JACC 2(6):1107-1113 (December 1983); Josephson, et al., "Recurrent Sustained Ventricular Tachycardia," Circulation 57(3):440-447 (March 1978); and Scheinman, et al., "Catheter Ablation of the Atrioventricular Junction: A Report of the Percutaneous Mapping and Ablation Registry," Circulation 70(6): 1024-1029 (December 1984).
Normally, the heart possesses an intrinsic pacemaker function in the sinoatrial (SA) node, which is in the right atrium, adjacent to the entrance of the superior vena cava. The right atrium is one of four proper anatomic chambers of the heart. The other chambers are the right ventricle, the left atrium, and the left ventricle. The superior vena cava is a major source of venous return to the heart.
The SA node possesses an area of specialized cardiac tissue called Purkinje cells which usually measures about 11/2 centimeters by about 21/2 millimeters. An electrical impulse normally exits from the SA node and travels across the atrium until it reaches the atrioventricular (AV) node. The AV node is located in the right atrium near the ventricle.
Emerging from the AV node is a specialized bundle of cardiac muscle cells which originates at the AV node in the interatrial septum. This "bundle of His" passes through the atrioventricular junction and later divides into left and right branches which supply the left and right ventricles. The left and right bundles further give rise to branches which become the so-called distal His-Purkinje system, which extends throughout both ventricles.
In a normal situation, therefore, an impulse originates intrinsically at the SA node, transmits through the atrium and is modified by the AV node. The AV node passes the modified impulse throughout the left and right ventricles via the His-Purkinje system to result in a coordinated heartbeat at a normal rate. Many factors affect the heart rate in addition to the intrinsic conduction system. For example, normally the heart rate will respond to physiologic parameters such as stress, exercise, oxygen tension and vagal influences.
Abnormal tachycardia, can arise from a number of causes. One group of such causes relates to abnormalities in the heart's conduction system. For instance, an ectopic or abnormally positioned electrical focus may take over the normal function of a node such as the SA or AV node. In addition to abnormal automaticity, arrhythmias may develop on the basis of accessory pathways bridging atrium with ventricle. Such pathways act as short circuits serving to maintain abnormal rhythms.
Treatment of abnormal tachycardias arising from ectopic foci or so-called ectopic pacemakers can include pharmacologic therapy or ablative therapy. The latter refers to destruction of the tissue responsible for the genesis and perpetuation of the arrhythmias. The ablative therapy may be accomplished by percutaneous insertion of a catheter or by surgical cardiac procedures.
Normally, when performing a percutaneous or closed chest cardiac ablation procedure, fluoroscopy is used to visualize the chambers of the heart. Fluoroscopy uses roentgen rays (X-rays) and includes use of a specialized screen which projects the shadows of the X-rays passing through the heart.
The technique of percutaneous ablation includes cardiac catheterization. A catheter is placed in an artery or a vein of the patient depending typically on whether the left (ventricle and/or atrium) or right (ventricle and/or atrium) side of the heart is to be explored and portions thereof ablated. For example, the left atrium can be entered by forcing the catheter through the interatrial septum after advancing the catheter from a vein and into the right atrium.
Frequently, an artery or vein in the groin, such as one of the femoral vessels, is selected for catheterization. The catheter is passed within the blood vessel and then into a vena cava or the aorta, depending whether a vein or an artery was entered, respectively, and from there into the appropriate atrium and/or ventricle. Vessels in addition to the femoral artery and/or vein can be used. Examples include the jugular, antecubital and subclavian vessels.
The catheter is steerable and it is positioned close to an endocardial or intravascular region of interest. "Endocardial" refers to the inner lining of the heart. An intravascular region can be of interest in part because the better approach anatomically to a cardiac electrical anomaly is sometimes by way of a vessel such as the coronary sinus.
The catheter includes electrodes as a means for sensing the electrical impulses originating in the heart. Thus, the electrode catheter can provide a number of readings from different areas of the internal aspects of the heart chambers proper and certain vessels, such as the coronary sinus, whose location provides an approach to the abnormally functioning cardiac tissue.
These various electrical recordings are correlated to provide an electrophysiologic map of the heart including notation of normal or abnormal features of the heart's conduction system. Once the electrophysiologic map is produced, an area can be selected for ablation.
For background information concerning catheter ablation and mapping techniques, see generally Scheinman, Melvin M. and Jesse C. Davis, "Catheter Ablation for Treatment of Tachyarrhythmias: Present Role and Potential Promise," Circulation 73(1): 10-13 (January 1986) and Kutcher, Karen L., "Cardiac Electrophysiologic Mapping Techniques," Focus on Critical Care 12(4):26-30 (August 1985).
Conventionally, a single catheter includes both mapping and ablation capabilities. For instance, an electrode catheter generally has four electrode tings positioned toward the distal end for mapping and a large electrode tip at the distalmost end of the catheter for RF ablation.
Steerable mapping and ablation catheters using RF energy are known. See, for example, U.S. Pat. No. 4,945,912 to Langberg. The RF energy is directed to the area to be ablated and causes tissue destruction by heat. RF ranges from 10.sup.4 Hertz (Hz) to 3.times.10.sup.12 Hz. Generally RF as used herein refers to the range of from about 100 kilohertz (KHz) to about 1 million Hertz (MHz). Additionally, direct current electrical, laser or microwave energy or cryoablation can be used instead of RF energy. See, for example, U.S. Pat. No. 4,641,649 to Walinsky.
Alternatively, ethanol has been infused into coronary arteries to ablate a focus such as a ventricular arrhythmia focus or the AV node. Unfortunately, this tends to result in a fairly large region of cardiac tissue death or myocardial infarction. With transarterial infusion, there is difficulty in precisely controlling the location and extent of the ablation.
Some of the problems with conventional fluoroscopic positioning of catheters include prolonged radiation exposure, sometimes as long as two hours. Additionally, the clinician may be unable to determine precisely where the catheter is in terms of the endocardium, vessel orifices and cardiac structures, such as valves.
During conventional ablation of cardiac tissue in attempts to destroy arrhythmogenic foci or accessory pathways, pathways causing AV node reentry, and ventricular tachycardia foci, it is difficult to precisely position the ablative function to correspond as closely as possible to the electrical abnormality identified by the mapping procedure. For example, blood flow, cardiac contractions and respiratory motion can alter the position slightly or destabilize the ablative function.
In summary, catheter ablation is applied to patients with worrisome cardiac rhythm disturbances. Current technology involves both (1) the use of mapping catheters to locate areas of cardiac tissue giving rise to electrical abnormalities and (2) the application of radio frequency current to or close to these areas for their destruction. In many situations the abnormal areas are readily found but stabilizing the ablation catheter is difficult because of a peculiar anatomic location or motion of the heart among other factors.